1. Field of Invention
This invention discloses an optical imaging system design concept that supports close to a two pi steradians solid angle field of view or any subset solid angle thereof using one optical system with multiple flat image collection devices. The optical design maintains high resolution over the entire two pi steradians solid angle field of view and minimal spatial distortion.
2. Description of Related Art
There are many surveillance applications, for example, but not limited too: commercial surveillance, security situations, traffic control, robotics, and military applications. Desirable features are wide field of view, high resolution over the wide field of view, and a non mechanical system.
There are three general approaches to this surveillance requirement, a mechanical scanning system, a wide field of view optical system, with an optically compressed image to a smaller field of view, or multiple optical systems. The latter two approaches being static collection systems, preferred over the scanning mechanical approach because of simplicity and reliability.
A major advantage of mechanical systems is that they offer a wide field of view with high resolution. The disadvantages are that mechanical systems are typically bulky, provide instantaneous limited field of view and subject to failure because of moving parts. These systems provide wide field with potentially high resolution over the wide field but at the cost of limited integration times and image blur for area arrays, and limited revisit times.
Another group of mechanical scanning systems use time delayed integration with charge coupled devices. Image blur is mostly overcome, and integration times are longer. The main disadvantage is the mechanical system.
Static surveillance systems have been preferred over mechanical systems because of simplicity and higher reliability.
One class of static systems use a single optical system to optically compress a wide field of view image into a narrow field of view image that fits the image collection device. These existing wide-angle surveillance systems use several optical compression techniques, which can be categorized into fish eye lens, complex refractor optical systems, parabolic reflectors or compression using a tapered fiber plate. These systems optically compress the wide field of view to fit image collection's narrow field of view, at the sacrifice of resolution. The images are not natural as seen by the eye having major spatial warp or fish eye effect, which can be compensated by image processing. However, the resolution at the periphery cannot be compensated, thus degrading the image resolution over the wide field of view.
Another class of static systems is the multiple optical systems. These designs use multiple optics each with it's own optical system and image collection device. The independent images are stitched together using software to provide a single image and correct for image residual distortion. The multiple optical systems drive complexity, size and cost. Electronics has miniaturized, but the optics has become more complex and has not reduced in size.
A serious optical design issue shared by all systems is that the natural focal surface is curved whereas the image collection devices are planar. The natural focal surface is spherical because of the physics of spherical lenses and refraction. Flattening and non spherical lens combinations are added to the optical designs to compensate and flatten the focal surface, which complicates the optical design and manufacturing costs. Curved image collection devices such as film on a curved surface have partially addressed this issue; however, most electronic image collection devices are typically flat.
Fiber products offer a simple way to map an image from a spherical focal surface to a planar detector surface. There are three fiber assemblies that are used to map the image from the spherical surface to a flat planar surface. These three assemblies are, (1) mineral fiber plates, which can be synthetically grown, (2) artificial fiber plates also referred to as fiber faceplates, and (3) fiber cables.
Mineral fiber plates are minerals such as ulexite that have a crystal growth forming parallel optical fibers that transmit light. These minerals thus map an image from one end of the crystal to the other without image distortion. Ulexite is commonly called TV rock in recognition of this phenomenon. Many minerals exhibit this phenomenon, and can be synthetically grown.
Artificial fiber plates are very short fused fiber bundles manufactured by laying the individual clad fibers parallel as possible, then fusing the matrix. Image distortion cannot be avoided, and the longer the length, the more image distortion. These optical elements are also referred to as faceplates, and most commonly used as a window over detector arrays. The short thickness avoids noticeable image distortion. The cladding introduces some obscuration and can also introduce a chicken wire effect on the image. Current artificial fiber plates advertise 70% throughput based on cladding obscuration.
The third fiber assembly is a fiber cable, which is used in borescopes. These cables are made by winding a clad fiber in a circle, fusing one section of the fiber bundle, and slicing the bundle at the fused section. The fibers are thus coherent at the point where they are sliced, and any shortening of the fiber cable introduces image distortion such as forming one end of the fiber cable spherical. The fiber cladding introduces obscuration by blocking part of the image. Too much fiber cladding results in what is referred to as a chicken wire effect on the image.
U.S. Pat. No. 3,455,223 by Charles P. Spoelhof et. al., teaches a optical imaging design using four half ball lenses positioned with a common center as the imaging device, which provides a spherical image surface with an air gap between the image surface and the last half ball lens on the image side. Film is used as the image capturing device with the film curved to best fit the spherical image surface. The design predates electronic image collection devices, thus restricts usage to film.
U.S. Pat. No. 5,004,328 by Masayuki Suzuki et al. teaches the advantages of concentric refractive optical designs with examples.
Suzuki teaches an imaging device using monocentric spherical half and full ball lenses with the aperture stop in the center, and a fiber bundle to translate the image from the spherical image surface to a flat detector.
The fiber's numerical aperture limits the field of view. To accommodate the fiber's numerical aperture limitations and achieve a wider angle, the fibers at the periphery of the image are positioned perpendicular to the spherical image surface.
Suzuki does not provide discussion on how to maintain avoid image distortion in the custom fiber bundle. Positioning fibers peripherally and maintaining image coherence at both ends of the cable is a very difficult and labor intensive operation, thus conceptually understandable, but in practice extremely difficult to achieve, resulting in an impractical solution. Suzuki does not consider multiple fiber plates juxtaposed with multiple planar detectors to provide a wider field of view. Suzuki does not identify fiber plates either artificial or natural but only identifies artificial optical fiber cables. The concept of mapping the curved focal surface to a planar image collection device using fiber cables is not practical because fiber cables are not coherent over their length, but only at the position they are sliced. Any reduction in length introduces image distortion, such as shaping one end spherical. Leaving the fiber bundle planar will result in high out of focus.
U.S. Pat. No. 5,012,081 by Douglas R. Jungwirth et al. teaches a limited wide angle star tracking device using monocentric spherical half and full ball lenses in a refractive design with the aperture stop in the center and holographic lenses at the spherical detector surface. Jungwirth teaches the use of multiple planar detectors placed on the spherical focal surface. The planar detectors on the spherical image surface will cause a significant out of focus display not suitable for an image; however, for points of light such as stars, the use of holographic lenses and image processing compensates.
Jungwirth teaches the use of fiber bundles to translate the image from the spherical image surface to a planar detector image surface. However, Jungwirth is using commercial fiber cables to coherently transfer the image from the image surface to a planar image surface. The image translation is not truly coherent as discussed above in U.S. Pat. No. 5,004,328 by Masayuki Suzuki et al.
Jungwirth does not consider natural fiber plates, which do not introduce any image distortion over their length, thus allowing spherical shaping.
Each fiber in the artificial fiber bundles has a very small active aperture because of the fiber cladding surrounding each active transmitting fiber. Jungwirth does not consider natural or artificial fiber plates with a higher ratio of active fiber aperture as compared to area between the fibers.
Jungwirth teaches the use of flexible fiber bundles to translate the image to a higher volume space. Jungwirth does not consider the use of fiber plates, and does not consider the inherent ability for fiber plates placed vertical to a spherical surface to translate the image surface to a higher volume area required to accommodate the detectors and electronics.
Jungwirth's refractive optical design limits the field of view for star tracker purposes rather than wide angle surveillance.
Jungwirth only considers an optical stop at the optics monocentric center.
U.S. Pat. No. 6,003,998 by Pierre St. Hilaire teaches a high resolution, wide angle reflective imaging device with use of multiple detectors and multiple juxtaposed commercially available fiber plates, referred to as faceplates, to transfer the image from the spherical image surface formed in free space to the planar image detectors.
Hilaire does not consider a refractive optical system and points out the inherent advantage of a reflective system not having any chromatic dispersion. However, a ball lens design can minimize the chromatic dispersion with careful design, thus minimizing the effect to acceptable levels.
Hilaire's reflective optical design has several disadvantages when compared to a reflective optical design. The reflective design has considerable central obscuration because the detector array and secondary mirror and potentially the electronics are located in front and in the center of the primary mirror, which creates a large instrument as compared to a refractive design. The reflective imager cannot be inconspicuously located because the secondary, and electronics must protrude from any surface whereas with a reflective design only the outer image side half-ball lens needs protrude from a surface.
A refractive system thus lends itself to inconspicuous packaging because all of the electronics and the image side of the optics may be hidden from view beneath a surface. The refractive imaging system can be easily installed onto an airframe because only the refractive object side lens need protrude outside the aircraft outer mold line.
The reflective telescope design is more expensive, and easily misaligned as compared to a refractive design. In the reflective design, a stiff structure is required to hold the primary, secondary and the sensors on the spherical focal surface. A refractive system can be a single unit with the half ball lenses acting as both the supporting mechanism and the active optical elements. Furthermore the fiber plates or faceplates in a refractive system can have the fiber plates mounted to the half ball image surface, which is the curved focal surface. Thus a refractive system lends itself to be smaller than the refractive design, and to be a single rugged unit with all of the optics, fiber plates and detectors a single unit without the requirement for a stiff supporting structure to maintain alignment.
Ball lenses are more easily manufactured and therefore less expensive than the reflective optical elements. Ball lens designs can also minimize chromatic and spatial distortion, which competes with the reflective design approach. The refractive system allows for miniaturization wherein only the diffraction limit is an issue.
Hilaire teaches the use of commercially available fiber optic faceplates to map an image from a spherical surface to a planar surface, but does not discuss specifically the origin or issues associated with faceplates. A fiber optic faceplate is an optical mosaic in which fibers less than an inch in length are mechanically positioned as parallel as possible, then fused together to form a vacuum-tight glass plate. Because faceplates are made up of millions of small fibers sealed together under pressure and at a temperature so high that glass becomes slightly viscous, they always present some degree of image distortion. Faceplates are also restricted in length, unlike natural fiber plates.
Hilaire does not consider natural fiber plates. The main disadvantages of a commercial fiber faceplates are their image distortion, limit on length and high expense.
The artificial fiber's cladding reduces the active fiber plate's active aperture, and can cause a chicken wire effect on an image; however, current fiber faceplate manufacturers advertise no chicken wire effect.
Naturally occurring fiber plates do not have any image distortion or chicken wire effect, examples of such minerals are ulexite, trona and halotrichite, which can be synthetically grown. These natural mineral fiber plates do not introduce image distortion over their entire length, and allow lengths much longer than an inch without image distortion.
U.S. Publication. No. 2010/0200736 A1 by Leslie Charles Laycock et. al., describes a method to translate a curved focal surface onto an image plane. FIG. 1 shows a ball lens, 11 with input radiation 1, focusing onto the ball lens's opposite surface where in a fiber plate, 16 is positioned. The image is formed on the surface of the ball lens, 11, at the interface between the ball lens, 11 and the fiber plate, 16. The fiber plate, 16, acting as a coherent fiber, translates the image from the curved focal surface to a flat focal surface where a flat imaging collection device, 20 is positioned. The collected image has some spatial warp by virtue of a spherical surface projected onto a plane.
Laycock's optical design is not practical for an imaging device because the ball lens index of refraction needs to be on the order of two which typically is not uniform glass and will cause image distortion from dispersion. High quality optical glass typically has lower index of refractions. The design also restricts the optical system design because the design degrees of freedom such as ball lens radius are determined by the index of refraction. The ball lenses index of refraction determines the ball lens size and the resultant field of view based on a imaging device size. Thus the design degrees of freedom are very restricted because in order to increase field of view, the detector must be enlarged. Another disadvantage of this design is not considering multiple image collection devices. The image collection device as shown must be large, or the ball lens must be small enough to accommodate the image device. A further issue with the design is the fiber plate's fibers numerical aperture disallowing acceptance of the radiation at high angles as shown in FIG. 1. Laycock addresses the numerical aperture issue by incorporating complex grating surface image collection surface on the fiber optic bundle, 16.
U.S. Publication. No. 2010/0200736 A1 by Leslie Charles Laycock et. al., shown in FIG. 2 describes an alternate approach to translate a curved focal surface onto a image plane by use of multiple optical systems, 90 and multiple fiber plates, 16, arranged at different angles, which addresses the fiber plate's numerical aperture issue. Multiple fiber plates, 16, are arranged at different angles to image onto a single image collection device, 20. The disadvantages of this design are the requirement for multiple independent lens systems, 90, and the requirement for a single large image collection device, 20.
U.S. Publication. No. 2010/0200736 design advantage is use of different radii for the half ball lenses, which allows a wide selection of optical materials and different physical optical sizes. A design disadvantage is use of a single fiber plate with a single detector. The fiber plate's numerical aperture limits the angular field of view. The optical stop's aperture is subject to the cosine effect for wide angles. Natural fiber plates are also not considered. The state of the art for artificial fiber plates causes a chicken wire effect over the image. The fiber cladding significantly reduces each fiber's micro effective aperture because only a small area is presented by the active optical fiber with the rest of the fiber's area being cladding which blocks the input radiation.
U.S. Publication. No. 2009/0303592 A1 by John Peter Oakley, describes lens systems used as a retro reflector wherein the lens system is composed of multiple symmetric ball lenses sharing a common center. The design approach allows freedom to select lower index of refraction glasses and allows the optics size to be determined by the glass selection. The design provides to the optical designer many degrees of freedom allowing a physical size to be determined in advance. The design allows the image focus on the outer ball lens surface or at an extended symmetric distance with an air gap. The disadvantage is that the patent only considers retro reflectors and does not consider this design as an imaging optical system. The design does not consider different radii for the second half ball lens, thus restricting the design degrees of freedom. The design fuses the entire surface between the two half ball lenses, thus requiring very similar coefficient of expansion materials for the two half ball lenses. The only area that is best optically fused is the very center of the assembly where a natural optical aperture in an optical stop assembly would be placed for an imaging system.
U.S. Pat. No. 6,320,703 B1 by Chungte W. Chen et. al., describes an imaging system composed of two half ball lenses fused together with the half ball lenses of different radii and the curved focal surface at some distance from the two half ball assembly. At the curved focal surface a fiber optic plate, referred to as a fiber optic relay, translates the focused image from the curved surface to a flat surface where an image intensifier is located. A disadvantage is that only a single detector is considered, which restricts field of view because of the fiber relay's numerical aperture. The numerical aperture reduces and cuts off the input radiation at high angles. Another disadvantage is not considering multiple fiber relays with their own image intensifiers, allowing for multiple image collection devices. The disadvantage of optically fusing the entire plane of the two ball lenses as described above also applies.
U.S. Pat. No. 5,311,611 by Richard A. Migliaccio presents a wide imaging ball lens design shown if FIG. 3 using two half ball lens 12 and 13 of different radii. The optical design provides a curved focal surface, 14 congruent with the half ball lenses curved surface. A fiber plate, 16, is formed to conform to the image curved surface, 14, with the opposite fiber plate end being flat and formed to the flat CCD. An optical stop, 30, is placed at the interface between the ball lens 12 and 13 to provide an optical stop. The optical stop, 30, is aligned so that the aperture is at the geometrical center of the two ball lenses 12 and 13. FIG. 4 shows the optical stop 30 with its aperture, 31.
Migliaccio does not consider multiple fiber plates with multiple detectors.
U.S. Patent No. 2002/0085278 A1 by Aleksandra Kolosowsky describes an optical faceplate used as a projection display. The patent teaches characteristics of ulexite, and identifies other fiber plate materials both artificially grown and naturally occurring. The patent teaches that fiber plates can be seamlessly positioned to form an image projection surface. The patent teaches that synthetically grown fiber plates are superior in quality free from impurities. These synthetically grown fiber plates are considered natural for the purpose of this patent in order to distinguish between man made fiber plates or faceplates which are referred to as artificial. Man made fiber plates introduce image distortion, thus the longer the fiber plate, the more image distortion.
Kolosowsky does not consider application of the fiber plate as a method to translate an image from a curved focal surface to a planer focal surface.
U.S. Publication. No. 2002/0096629 A1 by James Korein teaches a method to spatially compress an image in order to fit onto a detector by providing a tapered fiber in a fiber bundle. The design is restricted to use of artificial fiber cables. Fibers are assigned to a certain pixel, thus avoiding both image distortion and the image chicken wire effect. The taper degrades the image resolution by taking an area off of the image surface and reducing its size to fit the pixel. The requirement to assign a fiber to each pixel is straight forward in concept, but very difficult to achieve, thus not a practical solution.
Natural fiber plates have fibers much smaller than the diffraction limit, and are naturally parallel making the mineral a coherent fiber plate. Assigning a pixel to a particular fiber is unnecessary since several fibers may service a single pixel. The fiber diameters are on the scale of nanometers. The natural fiber plates do not introduce image distortion.
U.S. Publication. No. 5572034 by Andrew Karellas teaches a method to generate a seamless image for X-ray images by using two or more fiber plates and two or more optical detectors. Fiber plates on object side are seamlessly placed on the image surface, and used to channel the image to separate detectors on the image side of the fiber plate. Artificial fiber plates are only considered. In one version, only flat image planes and flat CCD planes are considered which does not naturally separate the detectors, and does not provide sufficient volume for the multiple detectors. Restated, this dilemma is caused because of the parallel plane geometry. To resolve this dilemma, the fiber plates are softened with heat and twisted to form a bend, which allows space for the second detector. A curved image surface would allow the fiber plates to have a natural different angle when all placed with their centers perpendicular to the surface. A radial distance from a spherical surface also provides more area based on the square of the radius rather than the linear relationship of the length of the fiber plates. A second version angles the adjacent fiber plates with respect to each other, which separates the detectors, but does not provide the volume as would be achieved by a radial geometry wherein each fiber plate were positioned vertical to a spherical surface.
The fiber plate design maintains a flat object side to the fiber plate. The possibility of forming the object side of the fiber plate to a curved images surface is neglected, possibly to avoid image distortion.
The application is concerned with X-ray imaging and does not consider the application to wide field imaging.
U.S. Publication. No. 5349180 by Arthur H. Vaughan, teaches a optical imaging design using multiple coherent fiber cables mapping the curved image surface to a single flat detector. The design teaches the natural optical stop provided by the fiber optics numerical aperture. The design will not provide a high quality image because of the chicken wire effect caused by the fiber cladding. Multiple detectors are also not considered, restricting the image resolution to the particular size of a given detector versus area coverage.
U.S. Pat. No. 5,159,455 by John D. Cox et. al., teaches a method to obtain a high resolution image by use of a composite of multiple fiber bundles and multiple sub-images. Cox does not use the natural advantage of the geometry wherein if the fiber plates are arranged perpendicular to a curved surface, the geometry generates volume for the sensors without the need for a tapered fiber plates. The design also uses tilted fiber plates that are not tapered, but neglects the geometric advantage of a curved focal plane and the capability to match the fiber plate to a curved focal plane.
U.S. Pat. No. 4,323,925 by Gurdon R. Abell et. al., teaches a method to obtain a high resolution image by use of multiple CCD sensors integrated with fiber plates. Fabricating the fiber plates with a larger input aperture, and forming the fibers to a smaller output aperture obtains the space required for the individual detectors. The individual fibers are tapered in nature accordingly.
The design presumes an image plane, neglecting the natural curved image plane gained without field flatteners. If the fiber plates are arranged perpendicular to a curved surface, the geometry generates volume for the sensors without the need for a tapered fiber plate. The design presumes an image plane, neglecting the natural curved image plane gained without field flatteners. The design does not take advantage of the capability to match the fiber plate to a curved focal plane.